1. Field of the Invention
The present invention generally relates to an apparatus and method for transmitting messages in a one-way communication system and, more particularly, to an apparatus and method for transmitting and randomly retransmitting multiple messages over a single channel in order to insure that at least one copy of each message is successfully transmitted.
2 Background Description
The need for multiple access strategies arises whenever a number of users have to share a single communication resource. This problem arises because it is either cost prohibitive or impractical to dedicate a communication channel to a particular user. Many such algorithms have been proposed and implemented for data transmissions in such a communication system. These algorithms can be classified into three categories including deterministic access, controlled access and random access.
The most common example of deterministic access is time-division multiplexing (TDM), where a portion of the outgoing time frame is allocated to each user TDM technique has been successfully implemented in, among others, geostationary satellite channels.
With controlled access, the users gain access to the channel either through a central controller (polling) or by passing control from one user to another in a decentralized fashion (token passing). These techniques are often used on a microwave channel where all users transmit on the same frequency.
Random access techniques allow users to transmit at will. These techniques employ various methods to resolve collisions that occur whenever two or more users transmit at the same time. One of the more common random access strategies is the ALOHA algorithm which resolves packet collision by having a central station recognize a collision and request the user to retransmit the message. This process is repeated until no collision or a given message is detected. The ALOHA transmission algorithm has been implemented in computer communication networks operating in a local-area network (LAN) environment.
All of the major access strategies discussed been used primarily in two-way (duplex) channels. In other words, some sort of acknowledgment is used to tell the user about the status of the transmission. However, many applications exist where unidirectional (simplex) transmissions could possibly meet all or most of the systems requirements, making the use of access strategies like the ALOHA algorithm both inefficient and expensive. These applications include, for example, home-shopping networks, video-on-demand controllers, and various alarm systems. The major problem that exists with simplex transmissions is that the transmitting party has no way of knowing whether the message was successfully received by a receiving station. Therefore, it is imperative for a system designer to develop a transmission scheme which would provide for a highly reliable message transfer in a one-way communication environment.
The problem of developing a traffic model for a unidirectional channel has been of interest to researchers for a number of years. One such model was investigated by Massey and Mathys, "The Collision Channel Without Feedback", which was developed for a situation where all users share a common communication resource but, because of the inability to synchronize their clocks, cannot transmit their data packets in a time-sharing mode. Additionally, due to the lack of a feedback link, they can never be sure of their individual packet transmission outcomes. This inability of users to synchronize transmissions forces them to employ random accessing. The model proposed in Massey and Mathys requires each user to have a protocol signal generator allowing them to transmit packets only during a time period determined by this generator to reduce packet collisions.
The scheme proposed by Massey and Mathys is not well suited to applications such as a home-shopping networks, for example, where each user needs to be able to initiate a transmission at any given time. In addition, most of these applications require a higher probability of successful message transmission than would be possible with a single transmission in cases of anything other than an extremely lightly loaded channel (e.g., a channel with low message arrival rate). One of the ways to improve the probability of successful message transmission is to introduce stochastically distributed message retransmissions into the channel protocol, for which no provision was made by the channel model in Massey and Mathys.
Another model for a collision channel without feedback was considered over twenty years ago by Huber and Shah, "Simple Asynchronous Multiplex Systems for Unidirectional Low-Data-Rate Transmission". Huber and Shah looked at a system consisting of many peripheral transmitters and a single central receiver with unidirectional information flow. The transmitters would send short messages consisting of their own addresses and a small number of additional information bits. The transmitters had no way of recognizing whether the channel was busy or not, and they were totally independent of each other. The information flow would be carried over a single binary channel consisting of a radio link. Huber and Shah investigated the transmitter repetition rate that should be selected for ensuring a maximum of correctly received messages, the effect of the average transmission rate on the behavior of the system, and the optimal strategy that each individual station should use when transmitting the message. Huber and Shah determined the number of transmissions for maximum data flow, as well as the probability that a message would be received correctly under optimum conditions. Finally, they postulated that some form of stochastic message distribution by each individual station was necessary in order to improve system performance.
It is worth noting that Huber and Shah were primarily concerned with determining the optimal average transmission rate which would maximize the (expected) total number of correctly received messages from all users during some observation period (which they considered to be the time interval during which all system users executed their transmission attempts). Huber and Shah made no attempt to develop quantitatively a retransmission strategy which would improve the chances of each user having at least one message received correctly when the observation period for that user does not perfectly coincide with the observation periods for all other users.